feat(D.2b): Step-0 chrome sprites confirmed + direct-RenderSurface upload path

Step-0 prove-out result: retail UI chrome sprites are RenderSurface objects
(0x06xxxxxx) that must be decoded DIRECTLY, not via the Surface->SurfaceTexture
chain GetOrUpload uses for world materials (which produced 1x1 magenta/garbage).
Added TextureCache.GetOrUploadRenderSurface(id, out w, out h) — Portal/HighRes
TryGet<RenderSurface> -> DecodeRenderSurface(palette:null) -> upload, separately
cached. This is the path UI chrome + (later) dat fonts use.

Confirmed the universal floating-window bevel is an 8-piece border + center fill:
  center  0x06004CC2 (48x48)
  edges   0x060074BF/C1 (10x5 horiz)  0x060074C0/C2 (5x10 vert)
  corners 0x060074C3..C6 (5x5)
Recorded in RetailChromeSprites.cs (edge/corner->position mapping is a best
guess pending the LayoutDesc 0x21000040 parse; visually confirmed at panel
render). The memory-note ids were right; only the decode path was wrong.

Temporary prove-out harness (added to GameWindow.OnRender) removed. proveout*.log
gitignored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -30,6 +30,7 @@ references/*
 /.superpowers/
 launch.log
 launch-*.log
+proveout*.log
 launch.utf8.log
 n4-verify*.log